For 18-year-old Jordan Thompson, it was the thrill of knowing he had just won the biggest match of his young career.

The opening day of qualifiers for the 2013 Australian Open will hold different memories for two of Australia's top tennis prodigies, even though both youngsters took significant strides at Melbourne Park on Wednesday.

Already one of the country's highly rated juniors, Kokkinakis was shoved into the national spotlight on 30 minutes notice last week when he represented the US as a Hopman Cup fill-in, playing doubles with Venus Williams and holding his own in singles competition against Fernando Verdasco.

On Wednesday, he was determined to add to his growing reputation and went a long way towards doing so after pushing American Steve Johnson to the limit in a dramatic three-set contest lasting three hours and 46 minutes.

Kokkinakis admitted he was "knackered" after the gruelling final set that went back and forth for 32 matches until the more experienced and more powerful Johnson finally broke the deadlock to take the decider 17-15 and complete a 6-4, 6-7 (5-7), 17-15 victory.

However the South Australian lost no admirers with his grit and ability to find something special when he needed it, saving match points in the second and third sets against a 23-year-old who reached the third round of last year's US Open.

Kokkinakis' body gave out on him late in the two-hour final set, with a groin injury prompting him to call for the trainer at 14-13 in the third.

"That's something I've get to get better at, conditioning-wise, I've got to get fitter," said Kokkinakis, who will now set his sights on the Australian Open junior title.

But while Kokkinakis drew the biggest crowd, it was Thompson's upset win over Frenchman Nicolas Renavand that stole the show.

Thompson, born in Sydney, described his 4-6, 6-2, 9-7 triumph as "unbelievable" and was still shaking during his post-match interview.

"I'm over the moon. It's my first big win, and my first ranking points as well," said Thompson, who will now take on American Ryan Sweeting in the second round.

Johnson was one of several Australians to progress, with 20-year-old Alex Bolt, 21-year-old Matthew Barton and 27-year-old Adam Feeney also notching impressive wins.

However Australia's other high-rated youngster, 17-year-old Nick Kyrgios, was knocked out by American Bradley Klahn 6-3, 6-2, while Brydan Klein, Sam Groth and Jacob Grills were also eliminated.